The FCC process has become well-established in the petroleum refining industry for converting low value high boiling range petroleum fractions into high value lower boiling products, especially gasoline, propylene and other light olefins.
In the FCC process finely divided solid catalyst particles promote cracking reactions by providing both the heat for the reaction and the catalytic activity. The finely divided form of the catalyst can be made to behave like a fluid (hence the designation Fluid Catalytic Cracking) and it flows in a closed cycle between a cracking zone (riser reactor) and a separate regeneration zone.
The reaction zone of an FCC unit generally consists of two parts; a riser reactor and an RTD to rapidly separate the catalyst and reaction products. The RTD system is generally housed in a reactor vessel for mechanical considerations, this vessel also contains other devices important for the operation of the process. Once separated from the catalyst, the reaction products are routed away from the vessel to be quenched and split into the desired fractions.
In the riser reactor, hot catalyst comes in contact with liquid oil feed causing it to vaporize and allow the desired gas phase cracking reactions to proceed and various vapor phase hydrocarbon products, as well as solid coke deposits on the catalyst, are formed. At the end of the riser reactor, a rapid separation of catalyst from hydrocarbon product is desirable to control the reaction time to avoid over-cracking the hydrocarbon vapors. Constraining hydrocarbon conversion time to the riser reactor is desirable as this zone is designed to ensure intimate mixing of the vapor and solid catalyst. Once the mixture leaves the riser reactor, less intimate contact can occur in the containment/separation vessel and undesirable thermal cracking reactions can occur which lead to the loss of valuable products and generation of low value by-products. Containing the hydrocarbon vapors within the RTD and routing them as directly as possible out of the system minimizes the residence time at high temperature that results in thermal degradation. It is also desirable to quickly and completely separate the hydrocarbons vapors from the catalyst to end the catalytic cracking reactions. Two stages of vapor-catalyst separation are required to achieve a very high catalyst recovery; the RTD is considered the primary stage of separation, the secondary separation step consists of multiple high-efficiency cyclones. During the primary separation, hydrocarbon vapor is separated from majority of the catalyst and leaves the RTD through a chamber connected directly to the secondary separation step. Separated catalyst flows down another chamber known as a dipleg at the lower end of the primary separator into the stripping bed. As the catalyst flows down the diplegs it entrains hydrocarbon vapor. Catalyst and entrained hydrocarbon leave the RTD and flow into a stripping zone where they are further separated. As the catalyst passes through the stripping zone the hydrocarbon vapor between and inside the particles are removed by counter current flow of stripping steam. Catalyst free of hydrocarbon vapors but fouled with solid hydrocarbon coke leaves the stripping zone and enters the regeneration zone.
The primary separation is performed by an apparatus known to those skilled in the art as a Reaction or Riser Termination Device (RTD) which is located at the outlet of the riser reactor. The primary separator in this case is known as the Riser Separation System (RSS or RS2) and is normally followed by a second stage of separation, which typically comprises of cyclones.
Following primary gas catalyst separation, the catalyst flows into a stripper bed below the RTD, where it is contacted counter currently with stripping gas to remove any remaining volatile hydrocarbons entrained with the catalyst. The hydrocarbon stripped catalyst, typically referred to as spent catalyst, containing solid coke deposits is passed to a catalyst regeneration zone, where the coke is burned off, and the catalyst activity is restored. The regeneration step releases energy and raises the catalyst temperature, after the coke deposits are burnt off, the hot regenerated catalyst flows back into the reaction zone. Hydrocarbon vapor separated from the catalyst, flows to a downstream distillation system for fractionation into several products. The FCC unit comprising of the riser reactor regenerator assembly is self-heat balanced in that heat generated by the burning of coke in the regenerator matches the heat required for feed vaporization and the heat for the cracking reaction.
Prior art riser separation systems typically have at least two separation chambers with accompanying diplegs and at least two circulations chambers for separation of gas and the catalyst material, respective. U.S. Pat. No. 6,296,812 to Gauthier et al. provides an apparatus for separating and stripping a mixture of gas and particles having an envelope comprising separation chambers and circulation chambers distributed in connection with a riser separation system. The upper portion of each separation chamber has an inlet opening communicating with the riser reactor and a middle zone for rotating the mixture in a vertical plane and a lower zone known as dipleg to collect the separated catalyst particles. Each separation chamber comprises two lateral walls which are also walls for the circulation chamber, at least one of the walls of each chamber comprising a lateral outlet opening for mixing gas and particles into the adjacent circulation chamber. The circulation chamber has two additional openings, one at the top connected to an gas outlet tube which further connects to the secondary separator and a lower opening to communicate with the stripper bed below. Application of this apparatus is to fluidized bed catalytic cracking of hydrocarbons in a riser.
The Gauthier et al. device has multiple separation and circulation chambers and each separation chamber has its own dipleg comprising a particle outlet opening communicating below the separation chambers with a stripper bed. In the Gauthier et al. device the riser vapor and catalyst mixture is forced to decelerate and change direction before entering the separating chambers through windows in the riser top taking a one-quarter (¼) turn before separating from each other. The vapor then enters the collection chamber after taking an additional 180° turn underneath the separation chamber's deflector. Catalyst flows down the separating chamber into diplegs, designed for low mass flux to maximize gas disengagement. This device is mainly used as primary separation device for catalyst and vapor for internal riser systems contained within the reactor/stripper vessel. Stripping gas and hydrocarbon vapors entrained from separation chamber diplegs into the reactor, enters the collection chambers through the lower conduit, mixes with the riser vapor from the separation chambers before entering the gas outlet tube/collector and then flows into cyclones—secondary separator for final gas/catalyst separation. The above-discussed system is associated with low catalyst collection efficiencies. The inlet to the separating chamber has a severe 900 turn from the riser top and provides only a ¼ turn for gas and catalyst to separate from each other resulting in low separation efficiency. The 90° change in direction creates a turbulent catalyst flow regime at the inlet and requires time to develop the necessary flow structure to achieve good separation of the cracked gases from solid catalyst particles. There is no connection between the separating chambers creating the potential for uneven pressure distribution resulting in uneven loading to each chamber, thus resulting in low separation efficiency.
Another type of riser separation system, such as, U.S. Pat. No. 4,664,888 to Leonce F. Castagnos, includes a deflecting device. The Castagnos patent is directed to a rough cut catalyst-vapor separator for fluid catalyst cracking risers which is located at the outlet of a riser and causes the oil-catalyst mixture to undergo a tight 180° downward turn. The centrifugal separator is equivalent to one-half a turn inside a cyclone and causes most of the catalyst to move to the wall. Most of the oil vapors are squeezed out away from the wall. At the end of the separator is a shave-off scoop positioned to divide a predominantly catalyst phase from a predominantly oil phase. The shave-off scoops conduct the catalyst phase away from the center of the vessel and deposit it near the vessel wall where its downward flow is continued under the action of gravity. The oil vapor phase continue its downward flow for a while but then must undergo a 180° degree turn and flow upward to exit the vessel through a series of conventional cyclone separators. However, the second 180° turn of the oil vapors can re-entrain the separated catalyst which negates the initial gas solid separation.
Castagnos also discloses an open semi-toroidal deflecting device and the gas/catalyst mixture exiting the riser impinge on the surface of the deflector wherein the catalyst particles are compressed against it and the separated gas phase is supposed to enter an open area below the edge of the deflector. As the gases separate from the particulate phase the solids tend to slow down and the effect of gravity negates the initial separation achieved. Any remaining compressed particulate phase flows onto a collection surface, the particles then flow down and off of the surface towards the wall of the vessel. The separated gases are supposed to flow up the conduits not re-contacting the particulate phase. As such, the pressure below the impingement surface and collection surface is higher than the pressure above them. This pressure differential forces gas not only through the conduits but also through the open area below the edge of the deflector and the collection surface, thus further negating the separation already achieved. Subsequently, the separated gas is “uncontained” in that it enters the vessel and experiences considerable residence time and undergo post riser cracking.
As such, there remains a need within the industry for a riser separation system having improved efficiencies. The present inventors have discovered a method and means to achieve improved catalyst and vapor phase separation, as well as improved gas collection efficiency, utilizing a riser separation system with a novel design providing an improved flow profile that promotes gas solid separation and improves operational stability.